1. Field of the Invention
The present invention relates to a compact surface-mount type antenna and an antenna apparatus for use in mobile communication equipment such as a cellular phone.
2. Description of the Related Art
In keeping with rapid advancement of down-sized mobile communication equipment such as a cellular phone, miniaturization has been underway in an antenna which constitutes such equipment. Thus, for example, a surface-mount type antenna has hitherto been developed. Now, a conventional surface-mount type antenna and an antenna apparatus incorporating it will be described with reference to a perspective view shown in FIG. 8.
In FIG. 8, reference numeral 50 denotes a surface-mount type antenna. This surface-mount type antenna 50 is mounted on a mounting substrate 56, thus constituting an antenna apparatus 61. In the surface-mount type antenna 50 shown in FIG. 8, reference numeral 51 denotes a substantially prismatic base body; reference numeral 52 denotes a feeding terminal; reference numeral 53 denotes a ground terminal; and reference numeral 54 denotes a radiating electrode. Moreover, in the mounting substrate 56, reference numeral 57 denotes a substrate; reference numeral 58 denotes a feeding electrode; reference numeral 59 denotes a ground electrode; and reference numeral 60 denotes a ground conductor layer.
In the conventional surface-mount type antenna 50, the feeding terminal 52 and the ground terminal 53 are formed on a side surface of the base body 51. The radiating electrode 54, which is routed as a slim conductor pattern, is configured as follows. At first it extends upwardly from the ground terminal 53 on the side surface, and is then substantially U-shaped, as viewed plane-wise, on a top surface of the base body 51 so as to take substantially the form of a loop, and eventually returns to the side surface once again to extend downwardly toward the feeding terminal 52. Moreover, the radiating electrode 54 has a gap 55 formed in a certain position thereof close to the feeding terminal 52. Thereby, the capacitance of the radiating electrode 54 can be so adjusted that impedance matching is achieved between the radiating electrode 54 and the feeding electrode 58 (feeding line) of the mounting substrate 56.
On the other hand, in the mounting substrate 56, on a surface of the substrate 57 are formed the feeding electrode 58, the ground electrode 59, and the ground conductor layer 60. The ground conductor layer 60 is arranged on one side of the ground electrode 59 so as to be connected thereto.
Then, the surface-mount type antenna 50 is mounted, with the feeding terminal 52 connected to the feeding electrode 58 and the ground terminal 53 connected to the ground electrode 59, on the surface of the mounting substrate 56, thus constituting the antenna apparatus 61.
In the conventional surface-mount type antenna 50, however, since the radiating electrode 54 is made short, there is a tendency of an operating frequency to increase. To decrease the operating frequency, the base body 51 needs to have a higher dielectric constant, or the radiating electrode 54 needs to be slimmed down.
However, an increase in the dielectric constant of the base body 51 gives rise to a problem of the antenna characteristics being abruptly changed to narrow-band characteristics. On the other hand, slimming of the radiating electrode 54 gives rise to a problem of great radiation loss.
Moreover, by adjusting a size of the gap 55 which is formed in the radiating electrode 54 to achieve impedance matching between the radiating electrode 54 and the feeding electrode 58, the impedance of the radiating electrode 54 can be changed. In this case, however, a resonant frequency of the antenna varies with the change of the impedance. This makes it difficult to attain the desired antenna characteristics as designed.
The invention has been devised in view of the above-described problems with the conventional art, and accordingly its object is to provide a surface-mount type antenna and an antenna apparatus capable of attaining satisfactory antenna characteristics with stability, of enhancing radiation efficiency, and of achieving miniaturization.
The invention provides a surface-mount type antenna comprising:
a base body made of a dielectric or magnetic material having a substantially rectangular solid shape;
a feeding terminal formed at one end of one side surface of the base body;
a ground terminal formed at another end of the one side surface of the base body;
a radiating electrode which has its one end connected to the ground terminal, the radiating electrode being disposed helically across the surfaces of the base body in such a way that it extends from the one side surface, across one principal surface, another side surface which is opposite to the one side surface, and another principal surface which is opposite to the one principal surface, and then returns to the one side surface and further extends, through the one principal surface, toward the one end of the one side surface; and
a wide-area portion,
wherein another end of the radiating electrode extends from the one principal surface, through the other side surface, toward the other principal surface, so as to form the wide-area portion facing the feeding terminal.
According to the invention, the radiating electrode is disposed helically across the surfaces of the base body in such a way that it extends from the one side surface, across the one principal surface, the other side surface, and the other principal surface, and then returns to the one side surface and further extends, through the one principal surface, toward the one end of the one side surface, and further the other feeding-terminal-side end of the radiating electrode extends across the surfaces of the base body, i.e., extends from the one principal surface, through the other side surface, toward the other principal surface, so as to form the wide-area portion facing the feeding terminal. With this configuration, the radiating electrode can be made longer, and also the wide-area portion of the radiating electrode can be electro magnetically coupled to the feeding terminal through an electric capacitance generated therebetween. Moreover, at the time of mounting on the mounting substrate, since a large capacitance can be created between the wide-area portion of the radiating electrode and the ground conductor layer of the mounting substrate, the resonant frequency of the radiating electrode can be decreased. This makes it possible to achieve miniaturization of the antenna without increasing the dielectric constant of the base body and without excessively slenderizing the radiating electrode.
Further, according to the invention, the impedance matching between the radiating electrode and the feeding electrode (feeding line) of the mounting substrate on which the radiating electrode is mounted can be achieved by adjusting the capacitance between the radiating electrode and the feeding terminal. The capacitance adjustment can be made by adjusting the configuration and/or area of the wide-area portion of the radiating electrode. Meanwhile, a dominant factor in the magnitude of the resonant frequency of the antenna is the capacitance between the radiating electrode and the ground conductor layer of the mounting substrate. Hence, variation in the resonant frequency resulting from the impedance adjustment by means of the wide-area portion can be minimized. As a result, it is possible to obtain a compact surface-mount type antenna that provides higher radiation efficiency and stable antenna characteristics.
In the invention, it is preferable that a width of the wide-area portion is adjusted to be three to ten times that of a conductor portion of the radiating electrode having a helical conformation.
According to the invention, the capacitance between the wide-area portion and the feeding terminal or the ground conductor layer can be increased, thus achieving satisfactory electromagnetic coupling with the feeding terminal.
In the invention, it is preferable that a length of the wide-area portion which lies on the other principal surface of the base body, extending from the other side surface-side to the one side surface-side, is determined such that the distance to the one side surface is equal to or greater than 1 mm.
According to the invention, it is possible to prevent occurrence of frequency variation which is caused by capacitance variation between the wide-area portion and the ground conductor layer resulting from antenna-mounting positional variation.
In the invention, it is preferable that the base body is made of a dielectric material having a relative dielectric constant xcex5r which is kept within a range from 3 to 30.
According to the invention, an effective length of the radiating electrode is decreased, and thus the current distribution region is increased in area. This allows the radiating electrode to emit a larger quantity of radio waves, resulting in advantages in enhancing a gain of the antenna and in achieving miniaturization of the surface-mount type antenna.
In the invention, it is preferable that the base body is made of a magnetic material having a relative magnetic permeability xcexcr which is kept within a range from 1 to 8.
According to the invention, the radiating electrode has a higher impedance, which results in a low Q factor in the antenna, and the bandwidth is accordingly increased.
The invention further provides an antenna apparatus comprising:
a mounting substrate formed thereon a feeding electrode, a ground electrode, and a ground conductor layer which is connected to the ground electrode and arranged on one side of the mounting substrate with respect to the ground electrode; and
the surface-mount type antenna of the invention as mentioned above,
wherein the antenna apparatus is constructed by mounting the surface-mount type antenna on the mounting substrate, with its other principal surface arranged on another side of the mounting substrate with respect to the ground electrode, and simultaneously connecting the feeding terminal and the ground terminal to the feeding electrode and the ground electrode, respectively.
The invention still further provides an antenna apparatus comprising:
a mounting substrate formed thereon a feeding electrode, a ground electrode, and a ground conductor layer which is connected to the ground electrode and arranged on one side of the mounting substrate with respect to the ground electrode; and
the surface-mount type antenna of the invention as mentioned above,
wherein the antenna apparatus is constructed by mounting the surface-mount type antenna on the mounting substrate, with its one principal surface arranged on another side of the mounting substrate with respect to the ground electrode, and simultaneously connecting the feeding terminal and the ground terminal to the feeding electrode and the ground electrode, respectively.
According to the invention, the antenna apparatus is constructed as follows. The surface-mount type antenna of the invention is mounted on the mounting substrate formed thereon the feeding electrode, the ground electrode, and the ground conductor layer which is connected to the ground electrode and arranged on the one side of the mounting substrate with respect to the ground electrode. Simultaneously, the feeding terminal and the ground terminal are connected to the feeding electrode and the ground electrode, respectively. With this structure, by adjusting the capacitance created between the radiating electrode of the surface-mount type antenna having the wide-area portion and the feeding electrode, ground electrode, and ground conductor layer of the mounting substrate, impedance matching can be achieved between the radiating electrode and the feeding electrode. Moreover, proper setting and adjustment of the resonant frequency and radiation efficiency of the radiating electrode, as well as miniaturization, can be achieved with ease. As a result, it is possible to obtain a compact antenna apparatus that provides higher radiation efficiency and stable antenna characteristics.
In the invention, it is preferable that the surface-mount type antenna is mounted on the mounting substrate at a distance of 0.5 mm to 3 mm from an end of the ground conductor layer of the mounting substrate.
According to the invention, the antenna apparatus is operable at a frequency band of 1 GHz to 10 GHz.
In the invention, it is preferable that the surface-mount type antenna is so mounted as to protrude from an edge of the ground conductor layer.
According to the invention, the bandwidth and gain of the antenna can be enhanced.
As described heretofore, according to the invention, it is possible to provide a surface-mount type antenna and an antenna apparatus capable of attaining satisfactory antenna characteristics with stability, of enhancing radiation efficiency, and of achieving miniaturization.